The invention relates to a method for setting up a useful data link between terminals in a VoIP system.
Communication networks whose components communicate with one another on a packet-switched basis are being increasingly used instead of line-switching communication networks. Networks such as these are also referred to in the literature as Voice-over-IP system (VoIP for short) networks, if the Internet Protocol (IP) is used for interchanging acoustic information in the form of data.
While telephones are generally used as terminals for line-switching telephony, the expression terminals is generally used in VoIP communication networks, because other devices, for example multimedia PCs, can also be used in addition to voice terminals, that is to say telephones.
The terminals in VoIP communication networks interchange data with one another during active operation. In this case, a distinction is drawn between control data and useful data. While the useful data in the case of voice terminals comprises the digitized speech signal, a control data is used to initiate and to control links between terminals. The service features which are also already known from line-switching-telephony, such as dialing, call back, call brokering, display indications and connection clearing, are provided by the control data in VoIP communication networks.
In “classical” line switching telephony, the terminals are not directly connected to one another, but are connected to a central switching instance, that is to say to a communication system. The control data sent by the terminals is evaluated by the communication system, and the useful data, that is to say the digitized speech information, is passed from the communication system to the respective terminal.
In the case of VoIP communication networks, on the other hand, the useful data is interchanged on direct paths between the terminals, while the control data is in most cases evaluated and passed on via a central instance. In networks such as these, this central instance is, for example, what is referred to as a gatekeeper, which, inter alia, carries out the conversion between call numbers and network addresses, and, thus, controls the setting up of links between the two terminals.
When interchanging control data and useful data, all the terminals, terminals and other network components in the communication networks have to comply with conventions (norms), which are defined in protocols and protocol families. By way of example, the Euro-ISDN Protocol E-DSS1 and the company-specific protocol CorNet-TS protocol are used in the line-switching communication network such as the ISDN, while the ITU-T H.323 protocol is widely used in VoIP communication networks. In this case, the only functions and service features which can ever be implemented in the communication networks are those which are defined in the protocol or protocol family that is being used. In this case, compliance with the conventions agreed in the protocols is important in order to make it possible to use components from different manufacturers and also to connect networks of different operators or different national states to one another.
While that part of the H.323 protocol family which defines the convention for transmission of the useful data in most cases corresponds to the requirements of the terminals that are used nowadays, the H.323 protocol elements for connection control (signaling) often do not (any longer) comply with the requirements. For example, these protocol elements do not allow the implementation of all the functions and service features which are known from the line-switching telephony protocols mentioned above. For this reason, not only is a control data channel set up on the basis of the H.323 protocol between the terminals and the gatekeeper (or the gatekeepers) in VoIP communication networks, but, in addition to this, a further controlled data channel is setup in parallel, which generally operates in accordance with a company-specific protocol and is required to provide functions and service features which are not defined in the H.323 protocol. This relates in particular to functions and convenience functions which are required for setting up links to modern terminals and which are not supported in the H.323 standard, such as short code dialing, extensive display indications and special calling and ringing tones. Thus, when setting up a connection which is initiated from one terminal, a control data link is first of all set up in accordance with a company-specific protocol between the calling terminal and the associated VoIP communication system with the gatekeeper functionality, and this VoIP communication system then sets up a control data channel to the calling terminal in accordance with the H.323 protocol.
The known method for setting up the link in VoIP communication networks has been found to have the disadvantage that the setting up of the H.323 link from the VoIP communication system to the terminal makes it impossible to use terminals which comply with H.323 standard to set up the connection in the opposite direction.